Neal Johns
Super Nomad
Posts: 1687
Registered: 10-31-2002
Location: Lytle Creek, CA
Member Is Offline
Mood: In love!
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What's this GPS Datum jazz??
If you really want to know, here is a sample of the story from:
http://www.cadapult-software.com/tutorials/cartography4.htm
....The first transcontinental arc of triangulation in the US was completed in 1899, connecting independent surveys along the Pacific Coast. In the
intervening years, other surveys were extended to the Gulf of Mexico.
The New England Datum was thus extended to the south and west without major readjustment of the surveys in the east. In 1901, this expanded network
was officially designated the United States Standard Datum, and triangulation station Meades Ranch, in Kansas, was the origin. In 1913, after the
geodetic organizations of Canada and Mexico agreed to base their networks on the United States network, the datum was renamed the North American
Datum.
By the mid-1920s, the problems of adjusting new surveys to fit into the existing network became clear. From 1927-1932 all available primary data were
adjusted into a system now known as the North American Datum of 1927 (NAD27). This datum served as the basis for detailed mapping and surveying for
many decades on this continent.....
My motto:
Never let a Dragon pass by without pulling its tail!
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David K
Honored Nomad
Posts: 65186
Registered: 8-30-2002
Location: San Diego County
Member Is Offline
Mood: Have Baja Fever
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Don't stop there Mr. Aridologist!
Now explain to the civilians why their brand new GPS is set to map datum WGS84    
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Neal Johns
Super Nomad
Posts: 1687
Registered: 10-31-2002
Location: Lytle Creek, CA
Member Is Offline
Mood: In love!
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Just click on the link given in the first post and all will become clear! 
(I didn't want to steal all the site's thunder)
My motto:
Never let a Dragon pass by without pulling its tail!
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mcgyver
Nomad
Posts: 444
Registered: 8-22-2003
Member Is Offline
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Neal, you sure read some complicated stuff!! Don't that hurt your head? Do mine!! No one has yet told me how my GPS knows that there is a McDonalds
at the next freeway exit when it only opened last week? And when I am running down Hiway 5 to Puertocitos it shows that I am sometimes a half mile out
in to the desert? Now I find out that all those WD23 and WS27 numbers and stuff are real and not something David K. thought up to hide the Baja
goodies!
Oh! Well I just find my way around the old fasioned way, put her in gear and head south till my feet gets wet!
Later people ! I am loading the trucks for BAJA!
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FrankO
Nomad
Posts: 301
Registered: 11-10-2002
Location: Ocean Beach
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Correct me if I'm wrong. Please.
I've been known to miss the obvious so you don't have to be gentle.
According to that article the WGS 83 is the more accurate setting for current mapping.
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Satellite Man
Junior Nomad
Posts: 90
Registered: 6-30-2004
Location: San Diego, CA
Member Is Offline
Mood: 
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I?ve been involved in the mapping field for about 10 years now and I still don?t have a firm grasp of the stuff. However, I can tell you this: for
most of us we don?t have to worry much about datums & ellipsoids. Even if you chose the wrong one you might be off by 20 to 300 feet. We can have
similar errors when reading a map. No matter how good of a ?map reader? you are you won?t be able to compensate for map warping and stretching.
Consumer grade GPS can also have errors associated with them (that?s even if you have perfect reception). I use professional grade GPS that has
sub-centimeter precision. However, it cost a couple of grand and you have to have some specialized software to process the data.
In short, most of us will be able to find what we are looking for without ?hurting our heads?
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Neal Johns
Super Nomad
Posts: 1687
Registered: 10-31-2002
Location: Lytle Creek, CA
Member Is Offline
Mood: In love!
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Want to play with different datum?
http://www.ngs.noaa.gov/cgi-bin/nadcon.prl
Want to find out the difference between NAD 83 and WGS 84? (this will fry your brain!)
http://www.mentorsoftwareinc.com/resource/Nad83.htm
[Edited on 9-30-2004 by Neal Johns]
My motto:
Never let a Dragon pass by without pulling its tail!
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